Ethylene glycol is an important basic organic chemical material and is mainly used to copolymerize with terephthalic acid to produce polyethylene glycol terephthalate (PET). In addition, ethylene glycol can also be used to produce antifreezing agent, lubricating agent, plasticizer, nonionic surfactant and explosive, etc, and it has a variety of uses. China is a big consumer of ethylene glycol. In recent years, with the constructions and operations of a number of large PET devices, the demands for ethylene glycol are increasing rapidly. Currently, the domestic output of ethylene glycol is far from being able to satisfy the demands. In 2010, China's import volume of ethylene glycol is 6.644 million tons, and it is estimated that by 2011, China's import volume of ethylene glycol will exceed 7 million tons. Hence, China's ethylene glycol industry has a good development prospect.
There are various process routes taking coal as the raw material to produce ethylene glycol, and one route having the best industrial prospect is to prepare oxalate through synthesis gas coupling and then produce ethylene glycol through the hydrogenation of oxalate. The reaction product obtained through the hydrogenation of oxalate to produce ethylene glycol comprises, in addition to the substances having lower boiling points such as methanol, glycollic acid ester, etc, a small amount of substances such as 1,2-propylene glycol and 1,2-butanediol which have the boiling points close to that of ethylene glycol and which can easily be subject to azeotropy with ethylene glycol and are hard to be separated through conventional rectification, wherein 1,2-butanediol has the boiling point closest to that of ethylene glycol and thus it is most difficult to separate 1,2-butanediol from ethylene glycol. Hence, the key of the problem is how to separate and remove 1,2-butanediol from ethylene glycol.
Besides, the route taking corn as raw material to produce ethylene glycol, 1,2-propylene glycol and 1,2-butanediol through biotransformation also draws the attention of various countries. To obtain various chemical alcohol products having high purities, including ethylene glycol and 1,2-butanediol, it still needs to solve the technical problem that conventional rectification requires many theoretical plates and large investment because the difference of the boiling points of ethylene glycol and 1,2-butanediol is too small.
There are few reports on the separation of ethylene glycol and 1,2-butanediol both at home and abroad. CN101928201 discloses purifying synthesis gas through the technical solution of saponification reaction, methanol-removal, hydrogenation reaction, three-column rectification and absorption treatment to prepare ethylene glycol crude product. The technical solution related in this patent does not separate 1,2-butanediol from ethylene glycol completely because 1,2-butanediol is subject to azeotropy with ethylene glycol during the process of separation and purification of three-column rectification. Moreover, it also causes product loss of ethylene glycol and reduces product yield. U.S. Pat. No. 4,966,658 discloses taking ethylbenzene, 3-heptanone and diisobutyl ketone, etc, as azeotropic agents to separate ethylene glycol and 1,2-butanediol or 1,3-butanediol by azeotropic rectification, and the number of theoretical plates of the rectification column is 30. However, the azeotropic agent related in this patent requires, during its use, a very high degree of vacuum (for example, 8 kPa) or a very long residence time (for example, 5-12 hours) in the case of lower degree of vacuum to obtain ethylene glycol with higher purity. And the content of ethylene glycol in the azeotrope at the top of the column is relatively lower, no more than 15%, and the resulting final product of ethylene glycol still contains about 100 ppm of 1,2-butanediol and a slight amount of azeotropic agents such as ethylbenzene, 3-heptanone and diisobutyl ketone, etc. Since these azeotropic agents have higher absorption in the optical ultraviolet region, the UV-transmittance of the product ethylene glycol is not high enough to meet the standard of superior grade product and thus said product is not suitable for industrial production.